How to bevel a flat surface?
ok, as @metaphor_set said, makes sense that bevel tool wont work in flat surfaces


  It still needs an angle, because otherwise a bevel doesn't make any sense


so, i go manually:



this measure addon helps a lot (edit>preferences>addons>"measureit")

[Edit] I found a new way to do this with exactly precision:

I used an plane as a knife to obtain a vertex at the intersection of the edge that I wanted to snap and now I can use the bevel tool. Here is the workflow:


[Edit 2] I found a faster way:

Using "Connect Vertex Path" (Ctrl+J) automatically create a new vertex at the intersection of the created edge. Here is the workflow: